According to the .NET Blog, .NET Framework 4.5 is an in-place update that replaces .NET Framework 4.0 (rather than a side-by-side installation). The goal is for .NET 4.5 to be fully backward compatible with applications built for .NET 4.0 (.NET 3.5 and .NET 4.5 will be side-by-side).

One of the first things you’ll notice about .NET 4.5 is the version number of the CLR (4.0.30319) is the same as .NET 4.0; this is the practice used by other in-place updates. (read more)

In fact, if you compare a system with .NET 4.5 with a system with .NET 4.0 you’ll see that 4.5 is just an update to 4.0: